图书馆毕业设

UseofVaporBarrierstoPreventCondensationWheneverinsulationisinstalledinawall,roof,orslab,itsresistancetotheflowofheatissomuchgreaterthanthatoftheotherelementsoftheconstructionthatthedewpointandresultingcondensationmayoccurwithintheinsulation.Sincewatervaporflowsfromregionsofhightemperaturetoregionsoflowtemperature,asimplesolutiontocondensationistostoptheflowofwatervaporbymeansofsomesurfacematerialimpervioustomoisture---providedthissurfaceiscalledavaporbarrier.Itmustalwaysbeappliedonthewarmside.Becausecondensationisgenerallymostsevereduringtheheatingseason,allvaporbarriersshouldbeinstalledontheinteriorsideofwallsandroofs.Fromapracticalstandpoint,thismeansthatthevaporbarriershouldbenexttoandpartoftheinsulation.Oneofthebestandmosteconomicalvaporbarriersisaluminumfoil.Someinsulationcomeequippedwiththisfoilattachedtoonesurface.However,unlessreinforcedwithkraftpaperorsomeotherstrongmaterial,thefoiliseasilyripped,torn,orpunctured,andsoisoflittlevalueasabarrier.Sincevaporbehavesasagas,avaporbarrier,tobeeffective,mustbeairtight,orasnearlysoaspossible.Butthisisoftenanimpracticalrequirement.Forexample,consideraroofwiththeinsulationabovethedeckandbetweenavaporbarrierandwaterproofroofing.Unlesstheinsulationisofafirmmaterial,theinsulationtoexpand,formingbubblesunderthewaterproofing.Duringthecoolnessofthenight,thebubbleswillcontract.Afteraseriesofsunnydaysandcoolnights,thebendingbackandforthofthesurfacemaydestroytheroofing.Onewaytopreventthisistoside-venttheroofinsulationsothecontainedaircanfreelyexpandandcontract.Thesideventsmust,however,beprotectedfromdrivingrain.Vaporbarrierscanbemadeofothermaterialsbesidesaluminumfoil.Therearealuminumpaints,plasticpaints,someplasticfilms,asphaltpaints,rubber-basepaints,asphalt,andfoil-laminatedpapers.Itmustberememberedthatwaterrepellentsurfacesarenotnecessarilyvaporbarriers,thatis,airtight.Toevaluateavaporbarrier,aunitknownasthepermisused.Itisdefinedasavapor-transmissionrateof1grainofwatervaporthrough1squarefootofmaterialperhourwhenthevapor-pressuredifferenceisequaltoqinchofmercury(7,000grainsequal1pound).Amaterialhavingavapor-transmissionrateof1permorlessisconsideredagoodvaporbarrier.Thecorrespondingunitforpermeanceof1-in.thicknessisperm-inch.Resistancetovaportransmissionisthereciprocalofthepermeance..Sincevaporsflowfromthewarmsideofawallorrooftothecoldside,theexteriorsurfaceshouldbeasporousaspossibleorventedandyetofferprotectionagainstpenetrationofrain.Thisisparticularlyimportantwith“blown-in”insulationasappliedtoframehouses,forwhichavaporbarriergenerallycannotbeinstalled.Thistypeofinsulationalsoinvolvesanotherprinciple,which,ifignored,frequentlyisthecauseofpeelingofpaintandleadstounnecessaryrepairofrainguttersthatdonotleak.“Blown-in”insulationissprayedintothespacesbetweenthestudsofframeconstruction.Theinteriorsurfaceisgenerallylathandplaster,orwall-board-----bothporous.Theexteriorisgenerallywoodsheathing,withshingles,clapboards,orstucco.Theheatresistanceoftheinsulationissuchthatduringthewinterthelocationofthedewpointfallswithintheinsulation.Theoretically,theresultingcondensationshouldoccurwithintheinsulation.This,however,doesnotoccur.Condensation,whenitwithintheinsulation,butontheinsidesurfaceofthesheathing.Theprincipleinvolvedisthis:Wheneverthedewpointoccurswithinamaterial,condensationwillnotoccuruntiltheflowofwatervaporencountersthesurfaceofanothermaterialofgreaterresistancetotheflowofwatervapor.Thatis,aslongastheaircankeeponmoving,itwillcarrythemoisturealongwithitandwillnotdepositthemoistureuntilitreachesasurfacethatresistsitsflowandiscolderthanthedewpoint.Theprobleminherentinblown-ininsulationcanbesolvedby“cold-sideventing.”Inapplyingblown-ininsulation,anopeningusuallyisdrilledthroughtheexteriorwallsurfacebetweeneachpairofstuds.Theseholesshouldneverbescaled,onlycoveredwithporouswater-repellentmaterialforprotectionagainsttheweather.Then,whateverwatervaporflowsthroughtheinsideporousfinishcanescapetothecoldairoutsidewithoutcondensing.Withclapboardconstruction,“toothpick”wedgesmaybedrivenundertheloweredgeofeachclapboardtoprovidetherequiredopeningsforbreathing.Tosumup:vaporbarriers,orasmuchresistanceaspossibletovaporflow(orair)shouldbeprovidedonthewarmsideofwallsandroofs.Openingsorporousmaterials---aslittleresistanceaspossibletovaporflow---shouldbeprovidedonthecoldside.Ifvaporbarrierswereperfect,cold-sideventingwouldnotberequired.Unfortunately,vaporbarriersarenotperfect;therefore,cold-sideventingisworthwhileinsuranceagainstfailureofinsulationinallcases.Thediscussionsaboveofwintercondensationseemtocontradictsummerrequirementswhenthewarmandcoldsidesofaconstructionarethereverseofwhattheyareinwinter.InmostpartsoftheUnitedStates,however,coolingseldomresultsinmaintenanceofinsidetemperaturesmorethan15`Fbelowoutsideconditions,whereasinwinter,insidetemperaturesategenerallymaintainedat60to75`Faboveoutsideconditions.Soinwinter,theprevailingmaximumtemperaturedifferencesarefromfourtofivetimeswhattheyareinsummer.Furthermore,insummerverylittlecoolingisrequiredduringthenight.Hence,asfarasinsulationisconcerned,summercondensationissointermittentthatitcanbecompletelydisregardedfortheaveragestructureandaverageoccupancy.Itshouldbementioned,however,thatinlow-temperaturework,suchascoldstorageroomsandlow-temperaturetestcellsspecialconditionsariseforwhichitisbesttorefertoaspecialist.使用隔汽层防止冷凝只要在墙、屋盖、或楼板内放置绝热层，由于它抵抗热的能力比其它构件大得多，在绝热层内可能形成露点和由此产生的冷凝现象。由于水汽从高温区流向低温区，解决冷凝的一个简单方法是用某种不透水的表面材料（只要它永远在露点以上）阻止水汽的流动。这种表面称为隔汽层。它应永远装在暖面。因为冷凝现象通常在采暖季节最为严重，因为冷凝现象通常在采暖季节最为严重，因此所有隔汽层都必须设在墙和屋顶的内侧。从实际的观点出，这意味着隔汽层应紧贴绝热层并构成绝热层的一部分。最好最经济的隔汽层之一是铝箔。有些绝热层事先有一面装有铝箔。但是除非有牛皮纸或其它结实材料加固，这种铝箔很容易被割裂、扯破、或穿孔,所以用作隔汽层没有多大价值.因为水汽的性质和气体一样,隔汽层必须不透气或尽可能不透气才能生效.但这往往不切合实际要求。例如，一个屋盖上的绝热层位于隔汽层和屋面防水层之间。除非绝热层是一种坚固的材料，如泡沫玻璃，否则太阳的热力将使绝热层中的空气膨胀，在防水层下形成气泡。晚上凉爽时，气泡将收缩。在一连串出太阳的白天和凉爽的夜晚之后，表面涨而复缩将会破坏屋顶。防止这种现象的一种方法是使屋顶绝热层有侧边透气孔，内部的空气能自由地膨胀和收缩。不过，侧边透气孔必须防止雨水渗入。除了铝箔，隔汽层还可用别的材料。有铝涂料，塑料涂料，某些塑料薄膜，沥青涂料，橡胶类涂料，沥青，和金属箔层压纸板。必须记住，防水表面不一定是隔气层，也就是说，不一定是不透气的。为估计隔汽层的优劣，使用了一种叫做perm的单位。其定义为：当水汽压力差等于1英寸水银柱时，每小时通过一平方英尺材料为一粒水汽的水汽传输率（7，000粒等于是磅）。水汽传输率为1perm或者1perm以下的材料就是优质隔汽层。渗透1英寸深的相应单位为1perm-英寸。抗水汽传输的能力是渗透能力的倒数。因为水汽从墙或屋顶的暖面流向冷面，外表面应尽可能多孔或通风，同时又要防止雨水浸入。对于构架房屋用的“喷吹”绝热层，这一点尤为重要，这种房屋通常不能设隔汽层。这种绝热层还涉及到另一原理，这一原理若被忽略了，常常引起油漆剥落，并导致不必要的修理雨水槽，其实它并不漏。“喷吹”绝热层被喷入